An optical head which is applied to an optical recording apparatus utilizing an optical disc is required to have functions of detecting a focus error to form a minute light spot on the optical disc and a track error to precisely trace a predetermined track on the optical disc. Further, the optical head is required to have a function of detecting an intensity of a light reflected from the optical disc to reproduce information stored in the optical disc, where the optical disc is a read-only optical disc, a write-once optical disc, or a phase-change optical disc.
In one type of a conventional optical head, a light reflected from an optical disc is partly introduced to a signal detecting system by a polarizing beam splitter, and is divided into a focus error detecting light and a track error detecting light in the signal detecting system by a beam splitter. Then, a focus error is detected in the knife edge method by receiving the focus error detecting light, and a track error is detected in the push-pull method by receiving the track error detecting light.
In a further type of a conventional optical head, a light reflected from a magnetooptic disc is partly introduced to first and second signal detecting systems by first and second polarizing beam splitters arranged in tandem on a light path. In the first signal detecting system, the introduced light is divided into first and second lights by a beam splitters, so that information stored in the optical disc is re-produced by a difference of light intensity between the first and second divided lights. In the second signal detecting system, the introduced light is also divided into third and fourth lights by a beam splitters, so that a focus error is detected in the knife edge method by receiving the third divided light, and a track error is detected in the push-pull method by receiving the fourth divided light.
In a still further type of a conventional optical head, a light reflected from a magnetooptic disc is divided into first and second diffraction lights by first and second birefringent diffraction grating elements having specified light axes and predetermined filling materials and arranged in tandem on a light path, so that information stored in the magnetooptic disc is re-produced by a difference of light intensity between the first and second diffraction lights.
According to the conventional optical heads, however, there is a disadvantage in that it is difficult that the optical heads become light and compact, because a number of parts such as a polarizing beam splitter, a half wave plate, a quarter wave plate, etc. are arranged in a predetermined designed pattern as explained in detail later. There is a further disadvantage in that it is difficult that the optical heads are decreased in cost and price, because the number of parts is large, the polarizing beam splitter is high in price, and the number of assembling steps is large as also explained later. There is a still further disadvantage in that a light utilizing factor is decreased especially in the optical head having the magnetooptic disc, because the first and second diffraction lights exist partly on a single light path, through which a light radiated from a light source is propagated in a direction to the magnetooptic disc.